CNC Machines

Look at Machine Capability Before Buying

Ervin, J Patrick

When selecting a machine tool, some companies want machines that minimize part cycle time, generally to the detriment of changeover time. Other companies want to purchase machines with a wide range of functionality-such as lathes with two spindles, multiple turrets or tool slides, live tooling, C axis, Y axis, and B axis. Generally these companies want to combine multiple machining operations on one machine to reduce the waste involved in handling parts between operations, while minimizing operator intervention and manufacturing throughput time. Other manufacturers want to purchase machines with limited functionality in terms of combining manufacturing operations, and elect to turn on a lathe, mill on a machining center, and grind on a grinding machine.

There is a wide array of improvement programs aimed at increasing the productivity of manufacturing operations. Lean techniques coupled with Six Sigma initiatives have proven their ability to increase productivity in company after company by focusing on and eliminating waste. But regardless of the manufacturing philosophy of your company, one thing often overlooked in the purchase decision is how machine capability, not functionality, affects waste on the shop floor. Functionality is simple. Once a decision has been made as to what level of functionality (combination of features) is desired by the company, elaborate spreadsheets are assembled comparing the features of brand "A" vs. brand "B," to determine which product is the best buy. Unfortunately, what generally occurs is that products are grouped by like features-factors like spindle horsepower, axis travels, and spindle speed.

All machines are not created equal.

This analysis is flawed. What isn't generally considered is a comparison of machine capabilities. Attempts are made to address the capability question by including terms on the spreadsheet such as axis repeatability and positioning error, turret-index repeatability, programming resolution, and spindle runout. These are all valid considerations, but to me capability is a machine's ability to produce a part to tolerance on an ongoing basis without significant operator intervention. Key differences exist between machine tools that may look similar on spreadsheets.

All machines are not created equal. We offer products for customers for whom price matters most, and we offer products for applications where performance matters most. Why don't two machines with similar functionality perform alike? The answer is not easily placed on a spreadsheet. Thermal effects can determine whether the first parts made in the morning are produced to tolerance or are scrap. Also, what occurs when the machine is stopped for a period of time during a shift? The difficulty of keeping a machine producing parts in-tolerance despite thermally induced changes in the machine structure is a factor not often considered.

What does a machine's capability mean to the level of waste generated on the shop floor? Obvious waste is the level of scrap or rework that must be performed. Another waste is the waste of an operator's time because he or she must tend to the machine and tweak the process to keep parts in an acceptable tolerance band. Waste can also be the waste of having to perform subsequent machining operations because the machine can't maintain the desired tolerance or surface finish.

These factors should be considered by manufacturing engineers when selecting a machine tool, but far too often the purchase decision is driven by price alone. Generally the cost of waste generated by an inferior machine tool over the machine's life will far outweigh its lower purchase price.

I don't mean that companies should over-spec their machines, but you must understand that all machines are not the same, even when similar specifications are listed on a spreadsheet. We offer machines with similar functionality that can differ in price by 20-80%. This price difference is based on changes in the machine structure that improve the machine's capability. The next time you contemplate a machine tool purchase, realize that the up-front price difference between two machines may be a minor point when compared to the impact on waste at the shop-floor level.

J. Patrick Ervin

PRESIDENT AND CEO

HARDINGE INC.

ELMIRA, NY

Copyright Society of Manufacturing Engineers Sep 2004
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